Bioprospecting South African essential oils’ metabolites for key enzymes of the ergosterol-biosynthetic pathway towards addressing postharvest fungal spoilage of tomato fruits

• Buchu essential oil is a possible promising natural alternative for the postharvest preservation of tomatoes. • Buchu oil exhibits significant antifungal activity against key spoilage fungi isolated from tomato fruits. • The likely antifungal mechanism of buchu oil is linked to the inhibition of enzymes within the ergosterol biosynthesis pathway • Lead bioactive compounds in buchu oil were identified through molecular dynamics simulations. Consumer demand for safe, chemical-free food products is steadily increasing, and this is also the case for fresh produce such as tomato fruits. In response to this, the present study isolates and molecularly characterizes spoilage fungi from two widely consumed tomato cultivars in South Africa. Six fungal strains were identified, and their susceptibility to buchu and sweet orange essential oils was evaluated through in vitro antifungal assays. Among the two, buchu oil (BO) shows stronger antifungal activity, with minimum inhibitory concentrations ranging from 2.22 to 35.59 mg/mL across the isolated strains. Subsequent gas chromatography–mass spectrometry analysis revealed that monoterpenes were the predominant bioactive constituents in BO. To elucidate the mechanism of antifungal action, these metabolites were computationally screened against three key enzymes in the ergosterol biosynthetic pathway (Erg11, Erg1, and Erg24) using molecular dynamics (MD) simulations. Notably, citronellyl acetate and diosphenol II showed strong binding affinity to Erg11 (−24.88 kcal/mol), citronellyl acetate to Erg1 (−32.29 kcal/mol), and limonene to Erg24 (−23.65 kcal/mol) with greater stability and compactness (RMSD: 1.88–7.97 Å; ROG: 23.19–25.75 Å) relative to fluconazole. Further validation of the reactivity of these lead compounds was achieved through density functional theory (DFT) calculations. Overall, the findings support the potential of BO as a natural antifungal agent and identify citronellyl acetate, diosphenol II, and limonene as promising candidates for the development of bio-based tomato preservatives. Nonetheless, further studies on the in vitro and in vivo antifungal efficacy of the identified metabolites are recommended to validate their practical application.